The present invention relates to a method for fabrication of a phase-shifter mask which facilitates stabilizing the size and the thickness thereof.
In fabricating a semiconductor integrated device by using photo-lithography, a designated pattern is formed on the photosensitive film by radiating ultraviolet light onto the surface of a wafer through a mask.
To accomplish such an exposure process, use of contact methods, proximity methods and projection methods are all well-known. However, because the line width of the circuit is narrower as the semiconductor device have tended to become super integrated, the exposure processes have exhibited certain limitations in their use.
In another well-known exposure process, a wafer stepping method which is capable of obtaining the narrower line width, has also been used. However, because the contrast of the pattern which is copied into a photosensitive film is degraded in the wafer stepping method, it can not be used in fabricating semiconductor devices of high integration.
Accordingly, a method had now been suggested which involves using an inversion mask which shifts the phase of the light passing through the edge 180.degree., when the light is passed through the designated pattern formed on the mask. Thus, there is a phase difference of 180.degree. at the edge of the designated pattern and the intensity of the light is 0 (zero), so that contrast at the edge portion is improved.
FIG. 1 shows a diagram illustrating a fabrication process for a phase-shift mask, according to the prior art, using poly methyl methacrylate (hereinafter, referred to as PMMA) as a phase-shifter.
The prior method for fabricating such a phase-shift mask is as follows.
First, as shown in FIG. 1(a), the Cr layer 30 as the mask plate and PMMA 20 of the photosensitive film as a phase-shifter are successively deposited on the quartz plate 10. Then, as shown in FIG. 1(b), PMMA 20 is exposed (or depicted) by the ultraviolet ray to form the designated pattern and, as shown in FIG. 1(c), the Cr layer is wet etched by using the said patterned PMMA as the mask for etching to form a Cr pattern.
According to the aforesaid method for fabrication of a mask, PMMA is patterned and then the Cr layer is wet etched, so that the thickness and size of the Cr pattern 30 of the mask plate become controlled and besides PMMA is used for a phase-shifter.
However, it is difficult to control the thickness of PMMA, which is regulated by viscosity of a solution and revolution speed in depositing. Therefore, as show in FIG. 1(c), the Cr pattern 30 has a trapezoid shape, so that contrast is degraded at the edge of the pattern.
Furthermore, it is very difficult to control the etching degree at the edge of the PMMA 20 and the endurability is not superior by using PMMA of the polymer system as a phase-shifter.